Many communications and other services are deployed over computer networks which include a combination of local area networks (LANs) and wide area networks (WANs). In many cases, such as voice over internet protocol (VoIP), the service relies upon the transmission of a real time data stream between two endpoint devices, such as VoIP telephone devices. The standard real-time transport protocol (RTP) is typically used to carry such media data streams.
To ensure quality of service (QoS), techniques have been developed for managing end-to-end performance between endpoint devices. Such management techniques may involve, for example, measuring, monitoring, recording, analyzing, and/or reporting the latency of the data streams as they pass between endpoints. Passive techniques are limited to recording packet traces and packet arrival times, while active techniques also inject synthetic traffic into the network.
To facilitate QoS measurements, RTP is often used in conjunction with RTP Control Protocol (RTCP). When both endpoint devices support RTCP, latency metrics such as round trip time (RTT) can be calculated using a passive technique, without the need to generate synthetic packets. However, some legacy endpoint devices do not support RTCP. As a result, passive techniques using RTCP observations alone can not be used in general for measuring latency between endpoints. To overcome these problems, active techniques may be used.
An example of an active technique is the PacketShaper® network management device by Blue Coat Systems, Inc. It includes a VoIP metrics feature to measure latency between two PacketShaper devices positioned near the endpoint devices. An active RTP flow between the endpoints causes the PacketShaper to generate its own RTP probe packets whose headers contain a timestamp, sequence number, and unit ID information. This header information is used by the PacketShapers to measure the round-trip time between the two PacketShaper devices. It is significant to note that this technique always requires the use of two PacketShaper devices, one at each endpoint. In addition, because the probe packets are generated throughout the entire duration of every active RTP flow, they can add as much as 5% to the network bandwidth requirements. Moreover, in the case where the endpoints do support RTCP, the synthetic RTP packets unnecessarily adds to network bandwidth overhead.